Glass is an inorganic product of fusion that has cooled to a rigid condition without crystallizing. The most common glasses are silicate glasses. The basic structural unit of silicate glasses has a silicon atom tetrahedrally-coordinated to four surrounding oxygen atoms. Similar to the crystalline silicates, the SiO4 tetrahedra in the silicate glasses may be found in a variety of configurations depending on the oxygen-to-silicon ratio in the glass compositions.
Some glasses are naturally occurring, such as, for example, perlite, pumice, obsidian, pitchstone, volcanic ash, and shirasu. Others, such as soda-lime glasses, may be produced synthetically. For example, soda-lime glass may be made by melting batches of raw materials containing the oxides of silicon (e.g., SiO2), aluminum (e.g., alumina (Al2O3)), calcium (e.g., CaO), sodium (e.g., Na2O), and sometimes potassium (e.g., K2O) or lithium (e.g., Li2O) in a furnace, and allowing the resulting melt to cool to produce the amorphous product. Glasses may be made in a wide variety of shapes, including sheets or plates, cast shapes, or fibers. Methods of manufacturing the principal families of glasses have been previously reported (e.g., Scholes, Modern Glass Practice, 7th ed. by C. Greene, Boston, Mass., CBI Publishing Company, Inc., 1974). Mineral wools, rock wools, and silicate cottons are generic names for manufactured fibers in which the fiber-forming substances may be slag, certain rocks, or glass (Kujawa, Industrial Minerals and Rocks, 5th ed., Littleton, Colo.: Society for Mining, Metallurgy, and Exploration, Inc., pp. 199-201, 1983).
Foam glasses are a class of lightweight glass materials having numerous sealed small cells. The base glass composition may be similar to typical window glasses, which may typically contain, for example, 70-73% SiO2, 1-3% Al2O3, 0.1-0.5% Fe2O3, 13-15% Na2O, 0-2% K2O, 5-7% CaO and 3-5% MgO (by weight).
Several techniques have been used to make foam glasses. For example, by leaching out the borate phase from a borosilicate glass, a silica-rich phase with very fine pores (e.g., 10 to 25 Å) may be obtained (see, e.g., Elmer, U.S. Pat. No. 3,592,619). Moisture trapped in the fine pores by leach solution may cause the fine pores to expand after heating the leached glass at, for example, 1,300° C.-1,425° C., by flash-firing. The foaming and sintering of the porous glass particles may occur generally simultaneously. Alternately, foam glasses may also be made by blowing air or other gases into molten glass and allowing the molten glass to cool and entrap the bubbles or cells in the solidified glass. However, these products have structural characteristics, such as low-compressive strength, low-abrasion resistance, and low-dimensional stability, which may not be desirable in chimney lining and structural applications.
In addition, it may be desirable to provide a ceramic material having thermal stability and low density. For example, it may be desirable to provide foam glass having a low coefficient of thermal expansion. Such foam glass may be desirable for, for example, lining chimneys, ductwork, inlets and outlets of scrubbers and smoke stacks for FGD (flue gas desulfurization) applications. Due to the high thermal expansion coefficient, regular glass tiles are often incapable of rapidly absorbing and evenly distributing heat during rapid temperature changes in the chimney and smoke stacks. The rapid temperature changes can cause the chimney lining tiles to change temperature much more rapidly on the inside than on the outside of the tile, resulting in an unequal expansion, which in turn, causes the flue tiles to crack and split apart.
Accordingly, there may be a desire to provide a more thermally stable foam glass having properties such as a low coefficient of thermal expansion (CTE), a low density, dimensional stability, abrasion and chemical resistance, and/or a high compressive strength.